Even though the well-written, and interesting, new question contains new information, I think what we are trying to achieve is to have a "timeless" question that can nevertheless have multiple answers posted for multiple versions, so that our combined knowledge over time about that question is collected as answers in the same place.
Ideally we would start with the "timeless" question that gets asked at a particular version, and then answers relevant to new versions can be added to it.
However, once the question has been re-asked, I think the action to take is to:
- Vote to close the new question as a duplicate of the old
- Perform any necessary editing to ensure that merging the questions will read as a "timeless" question with multiple answers that each clearly indicates the version(s) it refers to.
- Merge (or flag a moderator asking them to merge) the new question into the old
- Suggest the asker change their Accept checkmark to one about the later version, if that is the answer that now helps them the most, so that it may be brought to the top and be read first.
For the QGIS 2.6 and 3.0 questions linked at the beginning of this Meta question, I have already performed several small edits, to illustrate how the two Q&As may be made ready to be merged with minimal effort.